The use of emulsions is well known in the chemical art, including the pharmaceutical, specialty chemical and agricultural industries. In agriculture, emulsions provide appropriate formulations vehicles for delivery of herbicides, insecticides, fungicides, bactericides and fertilizers. Non-agricultural uses have included formulation of dyes, inks, pharmaceuticals, flavoring agents and fragrances.
Surfactants are required to aid in the emulsification process of oil into water (and vice versa) and to stabilise the thus formed emulsion against physical degradation processes.
A surfactant can adsorb to (and be desorbed from) an interface relatively easily. This process can lead to destabilisation of an emulsion; moreover, micellisation effects will also lead to redistribution of components throughout the system, often leading to Ostwald ripening and other undesirable interactions. All the many approaches to stabilisation of emulsions rely on one or more of the above described effects with typical surfactant adsorption as the stabilising mechanism.
Oil-in-Water emulsions (EW's) consist of a dispersion of oil droplets in a continuous aqueous medium. Such products are widely employed and encountered in various industries e.g., food (e.g. mayonnaise), detergency (e.g. removal of oil deposits), pharmaceuticals (e.g. drug administration), cosmetics (e.g. skin creams) and agricultural products (both as concentrates and diluted into water for application).
EW's are important in agriculture as a means of formulating oil-based systems in a more environmentally attractive form than the conventional Emulsifiable Concentrate (EC) where less solvent is sometimes possible per unit active ingredient and also as a precursor to Suspension-Emulsions (SE's) or Suspo-Emulsions which consist of a mixture of an oil-in-water and a suspension concentrate (SC). Such EW or SE products tend to have lower skin and eye toxicity ratings than the corresponding EC products as well as higher flash points and better compatibility with HDPE containers.
EW's, unlike EC's in the undiluted state, are only stable in the kinetic sense. This is because the system is inherently thermodynamically unstable and can only be formed non-spontaneously. This can be understood if one considers a large drop of oil, say with a volume of 1 ml, which is emulsified into many droplets each containing on average 0.001 ml. The interfacial area is greatly increased as a result of the sub-division of the bulk oil into much smaller units. This large interfacial energy is accompanied by a large surface energy that is given by the product of interfacial tension and increase in surface area ΔA (where ΔA=A2-A1 with A2 being the total area of the subdivided droplets and A1 that of the bulk oil). In the absence of any absorbed molecules at the interface the interfacial tension μSL is relatively large and hence the interfacial free energy for creating the interface ΔAμSL can be quite large. Thus the interfacial free energy opposes the process of emulsification.
It should be mentioned, however, that in an emulsification process, a large number of small droplets are formed and this is accompanied by an increase in the total entropy of the system. This increase in entropy facilitated the emulsification process although its value is relatively small compared to the interfacial free energy. From the second law of thermodynamics, the free energy of emulsification is given by the expressionΔGform=ΔAμSL−TΔSConf where ΔSConf is the configurational entropy term
In most dispersion processes ΔAμSL>>−TΔSConf and therefore ΔGform is large and positive. Thus the process of emulsification is non-spontaneous and hence with time the droplets tend to aggregate and/or coalesce to reduce the total energy of the system.
To prevent flocculation and/or coalescence one needs to create an energy barrier between the droplets to prevent their close approach. This energy barrier is the result of the creation of a repulsive force that overcomes the ever present van der Waals' attraction. The balance between repulsive and attractive forces determines the stability of the system against flocculation and coalescence. Assuming one can arrange to achieve a sufficient barrier to prevent close approach between the particles (i.e. stability in the colloid sense) what other factors play a role in keeping the droplets uniformly suspended in the continuous phase? One of the most important factors is gravity, which can cause separation of the droplets into a compact layer of cream depending on the density difference between the droplets and the medium, and their size.
There are basically five ways in which the structure of a dispersion of liquid droplets in a continuous medium can change. These are summarised as follows:
1. No change in droplet size (or droplet size distribution), but build-up of an equilibrium droplet concentration gradient within the emulsion. In limiting cases, the result is a close packed array (usually random) of droplets at one end of the system with the remainder of the volume occupied by the continuous phase liquid. This phenomenon results from external force fields, usually gravitational, centrifugal or electrostatic, acting on the system. “Creaming” is the special case in which the droplets collect in a concentrated layer at the top of an emulsion. A parallel effect may be seen when the oil phase has a density greater than 1.00 such that the cream sediments on the bottom of the container rather than rising to the top of the container when the density of the oil phase is less than 1.00.
2. Again, no change in basic droplet size or distribution but the build-up of aggregates of droplets within the emulsion. The individual droplets retain their identity. This process of flocculation results from the existence of attractive forces between the droplets.
3. In which flocculated droplets in an aggregate in the bulk of the emulsion, or alternatively, droplets within a close-packed array resulting from sedimentation or creaming, coalesce to form larger droplets. This results in a change in the initial droplet size distribution. The limiting state here is the complete separation of the emulsion into the two immiscible bulk liquids. Coalescence thus involves the elimination of the thin liquid film (of continuous phase) which separates two droplets in contact in an aggregate or a close-packed array. The forces to be considered here are therefore the forces acting within thin-liquid films in general. These can be complex and varied. In mixed emulsion systems, for example, in which there are droplets of liquid 1 and also liquid 2 dispersed in a continuous phase of liquid 3, coalescence between liquid 1 and liquid 2 droplets only occurs if liquids 1 and 2 are miscible with each other. If they are immiscible, then either droplet adhesion or droplet engulfment occurs. In either case, the thin liquid film between the contacting droplets is eliminated. When two droplets approach each other, surface fluctuations result since the interface is deformable. The amplitude of these fluctuations may grow to a considerable extent such that droplet coalescence occurs. However, this growth is opposed by the interfacial tension gradients that result in film expansion during growth of fluctuation. As a result of film expansion, regions of relatively higher interfacial tension than the rest of the film are created. This creates a gradient in the interfacial tension which tends to dampen the fluctuation. The driving force for this process is the Gibbs elasticity. The higher the Gibbs elasticity, the lower the tendency to coalesce.
Another factor which may retard coalescence is the surface viscosity which plays a role with many macromolecular films e.g. proteins. Indeed, such films are viscoelastic and they prevent coalescence by combination of the high viscosity in the film and elasticity which prevents fluctuation growth. Interfacial elasticity and viscosity may not be sufficient criteria for the prevention of coalescence, particularly when film drainage is fast. It is, therefore, essential to obtain information on the rate of drainage and equilibrium film thickness to assess emulsion stability.
4. An alternative way in which the average droplet size in an emulsion can increase, without the droplets coalescing, occurs if the two liquids forming the disperse phase and the continuous phase, respectively, are not totally immiscible. This is the case in reality because all liquid pairs are mutually miscible to some finite extent. If one starts with a truly monodisperse emulsion system, then no effects arising from this mutual solubility will arise. However, if the emulsion is polydisperse, larger droplets will form at the expense of the smaller droplets owing to the process known as Ostwald Ripening. In principle, the system will tend to an equilibrium state in which all the droplets attain the same size (this may be, of course, that state when we have just one single large drop). The process of Ostwald ripening results from the difference in solubility between small and large droplets. Since the solubility S of a particle of radius α is proportional to 2 μ/α where μ is the interfacial tension, then S is larger the smaller the droplet radius. Thus, for two particles with radii α1 and α2, where α1<α2, the solubility S1 is larger than S2 and,
            RT      M        ⁢    1    ⁢    n    ⁢                  S        1                    S        2              =                    2        ⁣        μ            p        ⁢          (                        1                      a            1                          -                  1                      a            2                              )      
where M is the molecular weight of the substance that has a density p. Thus the driving force of Ostwald ripening is the difference between S1 and S2. This means that, with time, the smaller droplets tend to dissolve and the solute diffuses in bulk solution and becomes deposited on the larger particles. This causes a shift in the particle size distribution towards the coarser size. This is clearly undesirable, since it accelerates sedimentation on the one hand and may produce a reduction in bio availability on the other.
5. A further way in which the structure of an emulsion may change is for the emulsion to “invert”, e.g. for an o/w emulsion to change to a w/o emulsion. This may be brought about by a change in temperature or concentration of one of the components or by the addition of a new component to the system. This may occur when the oil volume fraction exceeds a critical value Øcr that is usually the maximum possible packing fraction. For example, for a dispersion of uniform spheres, Øcr is 0.74 and any increase in Ø above Øcr usually results in inversion. Clearly with a polydisperse system, Øcr can exceed 0.74.
So the four main processes referred to above can be summarised as sedimentation (creaming), flocculation, coalescence and Ostwald Ripening. Certainly, in practical systems, all four processes may appear to occur simultaneously or sequentially in any order and this will depend upon the relative rate constants for the four basic processes under the conditions of storage of the emulsion.
Approaches to stabilisation of emulsions generally take the form of attempting to induce a repulsion between droplets by electrostatic or steric means—and this usually means the use of surfactants.
The best surfactants (emulsifiers) tend to be those of the block or graft type consisting of two main groups: the anchoring group B which must be chosen to have minimum solubility in the continuous medium and high affinity for the oil surface and A which must be chosen with maximum solvation by the continuous medium and minimum affinity for the oil surface. The ratio of groups A and B must be adequately chosen such that maximum adsorption occurs. It is clear that one should minimise micellisation of the block or graft polymer in order to allow adsorption to become more favourable. The length of the A chains must be optimised to give adsorbed layers with sufficient length such that the energy minimum seen in a typical energy interaction/distance curve for a sterically stablised particle becomes small. Various combinations of A and B groups may be produced of which A-B, A-B-A block copolymers and BAn graft copolymers are the most common.
It is also essential to choose materials that enhance the Gibbs elasticity. For this reason, surfactant mixtures, polymer/surfactant combinations, or liquid crystalline phases are most effective in producing stable emulsions.
To prevent sedimentation/creaming of emulsions it is essential to build up a “structure” (gel network) in the system that has (a) a high low-shear viscosity to overcome gravity and (b) sufficient elasticity (modulus or yield stress) to overcome compression of the whole network. Both can be achieved by the addition of a second phase that forms an “elastic” three-dimensional network in the medium. Several systems are available, of which xanthan gum (a microbial polysaccharide), sodium montmorillonite, microcrystalline cellulose and finely divided (fumed) silica are perhaps the most commonly used. Xanthan gum forms a highly elastic system as a result of polymer chain overlap. Sodium montmorillonite and microcrystalline cellulose form a gel structure as a result of interaction of extended double layers around the the thin clay platelet and/or edge-to-face flocculation. Fumed silicas form an elastic network as a result of the formation of chain aggregates. It is common to use mixtures of xanthan gum with sodium montmorillonite, microcrystalline cellulose or silica.
Various other methods may be used to build up a gel structure in emulsions. An example is controlled flocculation of electrostatically or sterically-stabilised dispersions. With electrostatically-stabilised dispersions, controlled flocculation is produced by addition of electrolyte which results in the formation of a sufficiently deep secondary minimum (of the order of 1-5 kT). This method must be applied with extreme care since coagulation may occur if the electrolyte concentration reaches a certain limit. With sterically-stabilised dispersions, controlled flocculation may be achieved by reducing the adsorbed layer thickness. Another method of controlled flocculation is that induced by the addition of a free non-adsorbing polymer to a sterically-stabilised suspension. Above a critical volume fraction of the free polymer (such as polyethylene oxide) weak flocculation occurs. This is usually referred to as depletion flocculation.
Much work has been carried out to produce “stable” emulsions as well as to more fully understand the processes whereby such emulsions deteriorate.
Approaches to the production of “stable” emulsions include:    1. Matching the densities of the oil phase to the aqueous phase (to minimise creaming or sedimentation) but this is difficult to achieve in practice due to variation of density with temperature as well as such a limitation resulting in low levels of active ingredient in a formulation.    2. Preparation of emulsions with a narrow particle size distribution. This is because a monodisperse emulsion cannot Ostwald ripen and clearly the narrower a distribution that can be achieved during processing, the less will be the drive to Ostwald ripen in the system.    3. Selection of the “best” surfactants to achieve charge-stabilisation, steric-stabilisation.    4. Use of colloid stabilisers such as polyvinylalcohol.    5. Stabilisation by adsorbed solid particles at the liquid/liquid interface, the so-called Pickering Emulsion.
It has been found that several of these approaches may have to be employed in a single formulation to achieve a storage-stable product. Even then, the products will only be kinetically stable (i.e. have a limited shelf-life-which may be two to three years) and given time will degrade.
A key mechanism of destabilisation is Ostwald Ripening, although it has received little attention in the public literature. Ostwald ripening is classically considered to occur because of the chemical potential difference between droplets (or particles) of different sizes and the transfer of oil from small to large particles such that the shape of the emulsion distribution and the size changes, moving upfield to a larger value with time. It has been considered that this transfer should be through the aqueous phase by dissolving oil in the continuous phase. In fact, water-insoluble oils do not “Ostwald” ripen when prepared as oil-in-water emulsions and this fact has been recognised in the literature.
However, until recently, it has not appeared to have been recognised that oils which are of a low water solubility and which might not be expected to Ostwald ripen quickly, can, in fact, do so rapidly, dependent upon the choice of surfactants employed to prepare and stabilise the emulsion. This is probably due to micelle transport of oil from small to large droplets. So we have, in most practical surfactant stabilised emulsions, two processes (at least) of effectively obtaining Ostwald ripening:
(a) Finite solubility in the aqueous phase of the dispersed oil,
(b) Ability of the oil to be dissolved in the surfactant micelle.
If either (or preferably both) of these processes can be prevented, then Ostwald ripening should not occur. This does not mean, of course, that a stable emulsion will then automatically be achieved, because other factors, especially the choice of surfactant type and amount, cosurfactant, temperature range for storage of the product then become critical in the overall formulation definition. However, if Ostwald ripening can be prevented, this would be a major benefit to all manufacturers and users of emulsions.
From the above, it is clear that a surfactant can adsorb to (and be desorbed from) an interface relatively easily. This process can lead to destabilisation of an emulsion; moreover, micellisation effects will also lead to redistribution of components throughout the system, often leading to Ostwald ripening and other undesirable interactions. All the many approaches to stabilisation of emulsions rely on one or more of the above described effects with typical surfactant adsorption as the stabilising mechanism.
If surfactants could be bound in such a manner that they could not be desorbed, then more stable emulsions could be produced, opening up new possibilities for processing of emulsions and uses for emulsions under adverse conditions.
Regardless of the type of process utilized, the final emulsion product may be packaged as a liquid containing oil droplets therein, or as a dried or solid formulation such as granules, tapes or tablets containing oil droplets which may be later added to a liquid solution such as a spray tank for agricultural use. This liquid often has various ingredients in addition to water, including wetters, dispersants, emulsifiers, protective colloids or colloid stabilizers and surface active agents or surfactants. The protective colloids serve to prevent agglomeration of the oil droplets. The surfactants perform various functions depending upon the type of surfactant used. These include varying the permeability of the wall, aiding in dispersing the emulsion, acting as a wetter, reducing or eliminating foaming, affecting the adhesiveness of the emulsion to the surface to which it is applied, and so forth. Primarily, conventional surfactants act as free, non-bound emulsifiers in the preparation of the emulsion.